1. Field of the Invention
The present invention relates to a sheet conveying system, as well as an image forming apparatus and a sheet conveying apparatus thereof, more particularly, to a sheet conveying system in which multiple sheet conveying apparatuses, each having a conveying roller pair which conveys sheets, are coupled together as in a configuration in which an image forming apparatus such as a copying machine or a printer is coupled with an optional apparatus, as well as an image forming apparatus and a sheet conveying apparatus thereof.
2. Description of the Related Art
In some sheet conveying apparatuses in which a plurality of conveying roller pairs nip and convey a sheet, a conveying roller pair on the downstream side may control acceleration, deceleration, or stop the conveyance of the sheets.
In such an apparatus, if the conveying velocity of the conveying roller pair on the downstream side is lower than that on the upstream side, a sheet being conveyed can be looped and physical damage to the paper, such as a bend, can be caused.
On the contrary, if the conveying velocity of the conveying roller pair on the downstream side is higher than that on the upstream side, both ends of a sheet is pulled in the opposite directions with respect to the conveying direction and the sheet would be torn.
To avoid these problems, techniques have been proposed including a technique that, while conveying roller pairs on the both of the upstream and downstream sides are nipping a sheet, a conveying roller pair on the upstream side has a one-way clutch so as to prevent the both ends of a sheet from pulling and a technique that controls to release the nipping of the sheet by separating the conveying roller pair on the upstream side (for example, see Japanese Laid-Open Patent Publication (Kokai) NO. 11-208939, No. 09-077299, and No. 05-270694).
However, the use of the one-way clutch for the conveying roller pair causes roller traces left on the surface of the sheet, which deteriorates image formed on the sheet.
Further, in the above conventional technique, the separation of the conveying roller pair on the upstream side is controlled having regard to only the fact that all the conveying roller pairs included in one sheet conveying apparatus are controlled by one controller in the sheet conveying apparatus. That is, the technique does not consider controlling separation of the conveying roller pair on the upstream side, in the case where a plurality of sheet conveying apparatuses each having a controller and a conveying roller pair are coupled together.
Further, it may solve the above problems that the velocity of the conveying roller pair on the upstream side is controlled so as to synchronize with the velocity of that on the downstream side. However, when the various types of sheet conveying apparatuses, each having sheet conveyance control means, are coupled together in various combinations, it is difficult to control each of the conveying roller pairs in these sheet conveying apparatuses to bring acceleration and deceleration of their conveying velocities into synchronization with each other.